Switching elements in modern packet communication networks commonly give priority to certain flows over others based on considerations such as differentiated quality of service (QoS) and congestion avoidance. (The term “switching elements” is used herein to refer to network elements having multiple interfaces to the network, such as network ports, through which packets are received and transmitted, and logic for directing each received packet from its ingress to its appropriate egress interface. Switches, bridges and routers are some examples of such switching elements. A “flow” is a sequence of packets transmitted through the network from a particular source to a particular destination.) In some networks, switching elements apply adaptive flow prioritization techniques, based on considerations such as the current queue lengths of different flows.
Techniques of this sort are described, for example, by Hoeiland-Joergensen et al., in “The FlowQueue-CoDel Packet Scheduler and Active Queue Management Algorithm,” published by the Internet Engineering Task Force (IETF) as draft-ietf-aqm-fq-codel-06 (Mar. 18, 2016). According to the authors, the algorithm that they describe (referred to as “FQ-CoDel”) is useful in fighting “bufferbloat” and reducing latency. FQ-CoDel mixes packets from multiple flows and reduces the impact of head of line blocking from bursty traffic, as well as providing isolation for low-rate traffic such as DNS, Web, and videoconferencing traffic. The algorithm is said to improve utilization across the networking fabric, especially for bidirectional traffic, by keeping queue lengths short.
Another flow prioritization technique is described in an Advantage Series White Paper entitled “Smart Buffering,” published by Cisco Systems, Inc. (San Jose, Calif., 2016). According to this White Paper, Cisco Nexus® switches use packet prioritization to provide latency benefits for small flows under load by automatically giving priority to the first few packets from each flow. A threshold is used to determine the number of packets that have been seen from a flow. If the number of packets received from the flow is less than the prioritization threshold, the packets are prioritized; otherwise, they are not. This mechanism allows short flows to have priority in both the switch and the network to reduce the number of drops, which have significantly greater impact on short flows than on long-lived flows.